1. Field of the Invention
The present invention relates generally to an electrostatic discharge protection devices, and in particular, to electrostatic discharge protection devices for protecting magnetic disk drives, such as a read/write transducer utilizing magnetoresistive sensors, against electrostatic discharge or electrical overstress during manufacturing and processing.
2. Description of the Prior Art
Magnetic head disk drive systems have been widely accepted in the computer industry as a cost-effective form of data storage. In a magnetic disk drive system, a magnetic recording medium in the form of a disk rotates at high speed, by means of a spindle drive motor for example, while a magnetic read/write transducer, referred to as a magnetic head, is suspended slightly above the surface of the rotating disk. The magnetic head is either attached to or formed integral with a flexure which is suspended over the disk on a spring-loaded support arm, known as a gimbal. An example of a magnetoresistive (hereinafter MR) head assembly 10 is shown in FIG. 1. MR head assembly 10 comprises, in relevant part, a gimbal 12 which supports an MR head 14 over the surface of a magnetic disk (not shown). The electrical impulses registered by MR head 14 pass through wires in gimbal 12 to a flex circuit 16. Flex circuit 16 has solder bumps 18 which are used to connect MR head assembly 10 to hard disk electronics (not shown) so as to allow reading from and writing to platters within the hard disk.
As the magnetic disk rotates at operating speed, the moving air generated by the rotating disk in conjunction with the physical design of the flexure operates to lift the magnetic head allowing it to glide slightly above the disk surface on a cushion of air, commonly referred to as an "air bearing cushion." The flying height of the magnetic head over the disk surface is typically only a few micrometers or less and is primarily a function of disk rotation, the aerodynamic properties of the flexure assembly, and the force exerted by the spring loaded gimbal.
One of the well-known problems encountered during the manufacture, handling and use of MR heads is the build-up and discharge of electrostatic charges on the various head components or on other objects which come into contact with the heads, especially thin-film heads. An electrostatic discharge (hereinafter ESD) occurs when electrostatic charge accumulates and builds up to eventually discharge to another surface of lower electrical potential. A sharp voltage spike caused by an electrostatic discharge can cause permanent and costly damage to individual parts of precision microelectronic devices.
Static charges may be produced by the presence of certain materials, such as plastics, during manufacture and subsequent handling of heads. These charges may discharge across the edge of the insulating layer between pole tips on an MR head, and adjacent conductive layers which are exposed and positioned adjacent to a transducing gap at the flexure air-bearing surface adjacent a recording medium. Such discharge may cause erosion to the pole tips and degradation of the transducer in reading and writing of data, or the discharge may destroy the MR head in its entirety.
Several solutions have been proposed to alleviate the problem of ESD on MR heads. For example, U.S. Pat. No. 5,465,186 issued to Bajorek et al., discloses an inductive head having short discharge pads formed by the deposition of conductive material in recesses formed in an insulating layer so that the electrostatic discharge will occur in areas displaced from the critical pole tip and gap area of the slider air-bearing surface. U.S. Pat. No. 4,800,454 issued to Schwartz et al., discloses an inductive head assembly wherein the magnetic pole piece in the inductive coil winding are coupled to the slider to allow discharge of any static electric charges which may build up. The winding is connected to the slider body via a diode in high forward and reverse voltage drops or through a fusible link.
MR heads are well known and are particularly useful as read elements in magnetic transducers especially at high data recording densities. The MR head provides a higher output signal than an inductive read head. This higher output signal results in a higher signal to noise ratio for the recording channel and thus, allows higher area densities of recorded data on a magnetic disk surface to be achieved. As described above, when a MR head is exposed to ESD or even a voltage or current input larger than that intended under normal operating conditions referred to as electrical overstress (hereinafter EOS), the MR read sensor and other parts of the head may be damaged. This sensitivity to electrical damage is particularly severe for MR read sensors because these sensors have a relatively small physical size.
For example, an MR head used for extremely high recording densities will have a cross section of 100 .ANG. by 1 .mu.m, or smaller. The discharge of only a few volts through such a physically small resistive area is sufficient to produce currents capable of severely damaging or completely destroying the MR head. The nature of the damage which may be experienced by a MR head varies significantly including complete destruction of the MR head via melting and evaporation contamination of the air-bearing surface, generation of shorts via electrical breakdown and milder forms of damage in which the head performance may be degraded. This type of damage to the MR head has been found to occur during both processing and use and poses a serious problem for manufacturing and handling of magnetic heads incorporating MR read sensors.
To accommodate testing of the MR head for exposure to ESD during production, test pads 20 may be provided on the end of flex circuit 16 for attachment to a test device. Physical alignment between test pads 20 and the test device may be accomplished by means of a registration hole 22. As solder bumps 18, test pads 20, and the wiring through the gimbal 12 remain exposed during assembly, there is a potential for electrostatic charge to accumulate readily and create a hazardous environment for MR head 14.
It has been found that providing an electrical short across the wires of the MR head element is effective in minimizing or eliminating damage from ESD as this method serves to raise the threshold voltage necessary to result in a failure. The shorted wires function by shunting most of the ESD current around the MR head element. Bajorek et al. '186 discloses shorting the MR head element wires at the sensor input pads. However, during assembly of the MR head, removal of the shorting elements becomes a difficult and expensive procedure to incorporate. Thus, there is a need for an electrostatic discharge protection device which provides electrical shorting of the head elements yet allows for easy removal before the MR head is assembled to the magnetic disk storage device.
Commonly-assigned patents that provide electrostatic discharge protection devices for various semiconductor packages include: U.S. Pat. No. 5,108,299 "Electrostatic Discharge Protection Devices for Semiconductor Chip Packages"; U.S. Pat. No. 4,971,568 "Electrical Connector with Attachment for Automatically Shorting Select Conductors Upon Disconnection of Connector"; U.S. Pat. No. 5,163,850 "Electrostatic Discharge Protection Devices For Semiconductor Chip Packages"; and U.S. Pat. No. 5,164,880 "Electrostatic Discharge Protection Device for Printed Circuit Board" all issued to David V. Cronin. Co-pending U.S. patent applications include Ser. Nos. 08/278024 and 08/278063 "Electrostatic Discharge Protection Device," both filed Jul. 20, 1994, and Ser. No. 08/234,917 "Electrostatic Discharge Protection Device" filed Apr. 28, 1994, all by David V. Cronin.
Accordingly, it is an object of this invention to provide a device which can be used to minimize damage to MR heads from ESD or from electrical overstress of the MR head or other transducer components.
It is another object of this invention to provide an MR head having a configuration such that excessive current is shunted away from the MR element and other critical head components.
It is a further element of this invention to provide an ESD protection device that automatically withdraws when coupled to either a testing device or a printed circuit board.
These and other objects of the invention will be obvious and will appear hereinafter.